This invention relates to microwave oscillators in which the frequency and power output thereof are controlled by double-mode tuning and more particularly to such a microwave oscillator in which characteristics thereof may be selected to provide optimum power output to a load impedance of any prescribed value. A double-mode tuned oscillator is one including structure supporting both odd and even mode fields which may be adjusted.
A prior art microwave oscillator is described in U.S. Pat. No. 3,699,475, which issued Oct. 17, 1972. Such an oscillator is also described in the article A Dual Mode Tuning circuit for Microwave Transistor Oscillators, IEEE Transaction MTT 25-2, Feb. 1977, pgs. 120-127. The oscillator in the patent comprises a single transistor and only a pair of parallel conductors having a pair of short circuits thereon which produce odd and even mode fields on the conductors. The collector and base electrodes of the microwave transistor there are connected to associated conductors, the emitter electrode being connected to a reference potential. An output signal is coupled from the collector electrode of the transistor. It has been found that the output admittance of that oscillator is complex, i.e., it includes a susceptance rather than being purely conductive. In order to obtain maximum output power from the oscillator, it is then necessary to either employ a complex load impedance which would have a different value for each frequency of oscillation or to employ an impedance matching network to transform the impedance at the oscillator output port into a pure resistance such as 50 ohms. Although this may be readily accomplished, it is inconvenient to have to do this.
An object of this invention is a provision of an improved double-mode tuned microwave oscillator in which the characteristics thereof may be readily selected so that its output impedance is purely conductive and is a prescribed value.